A portrait consists of two differently-sized graphics files in PCX format (256 colours only). One file is used for the larger portrait picture, and the other for the smaller one (such as the one that appears in the list of heroes opposite the list of towns on the main display).
The large file must be 58x64 (58 pixels wide and 64 pixels high).
The small file must be 48x32 (48 pixels wide and 32 pixels high).
Step 2:
Copy both files to the Hereos3\Data subdirectory
Step 3:
*Rename the files to match the internal names of an existing heroes portrait*
The name for the large portrait will be Hpl###xx.pcx where ### is a 3-digit code for the specific hero and xx is a two-digit code for the hero type.
The name for the small portrait will be Hps###xx.pcx.
You can replace any portrait with any new portrait. So you could replace a knight picture with a picture of a necromancer if you desire. It's up to you.
The hero code and number range is:
sh = special heroes (000-008)
el = elementalist (000-007)
pl = planeswalker (000-007)
kn = knight (000-007)
cl = cleric (008-015)
rn = ranger (016-023)
dr = druid (024-031)
al = alchemist (032-039)
hr = demoniac (056-063)
dm = heretic(048-055)<!--- be carreful, 3DO inversed
nc = necromancer (072-079)
ov = overlord (080-087)
wl = warlock (088-095)
br = barbarian (096-103)
bm = battlemage (104-111)
bs = beastmaster (112-119)
wh = witch (120-127)
?? = more special heroes (128-132)
The help page showing you exactly which hero has which number or you can just experiment yourself.
Examples:
To replace the 3rd necromancer hero portrait with a new one, name the two pcx files: Hpl075nc.pcx and Hps075nc.pcx.
These new files will sit in your data directory and can be removed, renamed or changed at any time.